The present invention relates generally to oscillators and, more particularly, to tunable oscillators having a transistor formed of a wide bandgap semiconductor material.
Upconverters and downconverters that include an oscillator, such as a voltage controlled oscillator, a phase locked oscillator or the like, are widely utilized for a variety of applications including signal transmission, signal reception and the like. With respect to commercial applications, upconverters and downconverters are utilized in broadband terrestrial and satellite communication systems, broadcast systems, radar systems and the like. For example, wireless radio systems include an upconverter for upconverting a low frequency baseband signal to a higher frequency for transmission purposes. Likewise, in military applications, upconverters are utilized not only as transmitters, but also as radar jamming devices and the like.
There are two principle types of upconverters for radio frequency (RF) systems, saturated upconverters and linear upconverters. As shown in FIG. 1, a saturated upconverter typically includes a phase locked oscillator 12 that receives a reference signal having a relatively low frequency and that produces an output signal having a higher frequency. As known to those skilled in the art and as depicted in FIG. 2, a phase locked oscillator typically includes a phase detector/discriminator 12a that compares the relative phases of a relatively low frequency reference signal and a feedback signal derived from the output of the phase locked oscillator. The output of the phase detector/discriminator is amplified by a loop amplifier 12b, filtered by a loop filter 12c and provided to a voltage controlled oscillator 12d in order to controllably adjust the output of the voltage controlled oscillator. In particular, the amplified and filtered signal is utilized to controllably alter the bias voltage applied to the transistor of the voltage controlled oscillator. The phase locked oscillator can also include an optional prescalar/divider 12e that modifies the output of the phase locked oscillator that is fed back to the phase detector/discriminator. As such, the phase locked oscillator forms a phase locked loop.
A saturated upconverter 10 also generally includes one or more drivers 14, one or more amplifiers 16 and a solid state power amplifier 18 for substantially amplifying the output signal provided by the phase locked oscillator 12. Although not depicted, the saturated upconverter can also include a frequency doubling element or the like for altering the frequency of the output signal. Once appropriately amplified, the output signal is provided to an antenna 20 for transmission.
As depicted in FIG. 3, a linear upconverter 22 also generally includes a phase locked oscillator 24, such as depicted in FIG. 2, for receiving a relatively low frequency reference signal and for producing an output signal having a greater frequency. A conventional linear upconverter also includes a first mixer 26 for combining an input signal having an intermediate frequency and a signal derived from the output signal of the phase locked oscillator. In this regard, the linear upconverter can include a frequency divider 28 disposed between the phase locked oscillator and the first mixer for reducing the frequency of the output signal of the phase locked oscillator. In addition, the linear upconverter can include one or more drivers 29 between the frequency divider and the first mixer in order to amplify the output of the frequency divider. Thus, the output signal of the phase locked oscillator, following its reduction in frequency and its amplification, serves as a local oscillator signal for the first mixer. The linear upconverter also includes a second mixer 30 for combining the output of the first mixer and another signal derived from the output signal of the phase locked oscillator. As depicted, the linear upconverter typically includes at least one and, more commonly, a plurality of drivers 32 and/or amplifiers between the phase locked oscillator and the second mixer in order to appropriately amplify the output signal of the phase locked oscillator. Although not depicted in FIG. 2, some linear upconverters include a frequency multiplier or the like between the phase locked oscillator and the second mixer for increasing the frequency of the output signal produced by the phase locked oscillator prior to its presentation to the second mixer. In any event, the signal derived from the output signal produced by the phase locked oscillator serves as a local oscillator signal for the second mixer. The linear upconverter can further include a solid state power amplifier 34 for amplifying the output of the second mixer prior to transmission by an antenna 36 or the like. Further, the linear upconverter can include filters, such as a first filter 38 disposed between the first and second mixers and a second filter 40 disposed between the second mixer and the solid state power amplifier, for blocking the respective local oscillator signals.
Both types of upconverters are effective for reliably producing RF signals of a predetermined frequency and power level. However, these upconverters are not as efficient as desired. For example, some upconverters produce RF output signals that have only about ten percent of the DC power that was input to the signal source. The relative inefficiency of conventional upconverters is attributable to a number of factors. However, one of the more prominent factors for this inefficiency is the DC power required to bias the plurality of components of the upconverter in order for the components to function as desired. In a saturated upconverter 10, for example, the phase locked oscillator 12, as well as each driver 14, each amplifier 16 and the solid state power amplifier 18 must be appropriately biased by means of a supply voltage and a supply current in order to produce the desired RF output signal. Similarly, in a linear upconverter 22, the phase locked oscillator 24, each driver 32 and the solid state power amplifier 40 must be appropriately biased by means of a supply voltage and a supply current in order to produce the desired RF output signal. As such, significant input power is consumed to appropriately bias each of these components, thereby substantially diminishing the efficiency with which these conventional upconverters operate.
Additionally, each component of a conventional upconverter occupies a certain amount of space. As such, upconverters that include a plurality of components, such as a plurality of drivers or amplifiers, will generally be somewhat larger. With increasing emphasis being placed upon the miniaturization of all electrical devices, including upconverters, the space requirements of each additional component of a upconverter disadvantageously limit the extent to which the size of a conventional upconverter can be reduced. Similarly, the bias circuitry required for each of these components requires some additional space, thereby further limiting the extent to which the size of a conventional upconverter can be reduced.
Although the drivers and amplifiers of a conventional upconverter decrease the efficiency of the upconverter and increase the space requirements of the signal source, conventional upconverters require drivers and amplifiers in order to appropriately amplify the signals provided by the oscillator prior to transmission. In this regard, the oscillators utilized by conventional upconverters are formed of traditional semiconductor materials having a relatively small bandgap, such as a bandgap of less than two electron volts (eV). For example, the oscillators of conventional upconverters are generally formed of silicon (Si), gallium arsenide (GaAs) or indium phosphide (InP) which have bandgaps of 1.1 eV, 1.43 eV and 1.34 eV, respectively. Transistors formed of these material systems can be readily fabricated and can offer extremely predictable performance. However, the oscillators that include transistors formed of these traditional semiconductor materials can not generally be fabricated in a cost-effective manner so as to produce signals having the power required for transmission. As such, the signals produced by the oscillators of conventional upconverters must generally be amplified, oftentimes repeatedly, prior to transmission. While each amplification stage increases the signal strength, the efficiency with which the output signal is produced is correspondingly decreased as described above.
Accordingly, while several types of conventional upconverters exist, a need persists for upconverters, both saturated and linear, to produce output signals of a desired power level and frequency in a more efficient manner. Additionally, it would be desirable for upconverters, both saturated and linear, to be designed in such a manner that the upconverters require less space and can therefore be more compactly packaged. Moreover, it would be desirable to provide upconverters, both saturated and linear, that required fewer components and less bias circuitry, thereby simplifying the design of the upconverters.
As depicted in FIG. 4, a downconverter 21, such as commonly utilized in signal reception applications, also includes an oscillator 23, such as a phase locked oscillator or a dielectric resonator oscillator. Although not illustrated, the phase locked oscillator would receive a reference signal, such as from a crystal oscillator or the like. The downconverter also includes a mixer 25 that mixes the received signal provided by an antenna 27 with the output of the oscillator. As shown, the downconverter also includes one or more low noise amplifiers 29 for amplifying the signals received by the antenna and one or more drivers and/or amplifiers 31 for amplifying the signals provided by the oscillator. The output of the mixer is then filtered by filter 33 and amplified by amplifier 35. As described above in conjunction with upconverters, the efficiency of the signal reception process is adversely impacted by the need to bias the various components, such as the drivers and/or amplifiers that amplify the signals provided by the oscillator. Moreoever, complexity of the design of a downconverter and the space required by a downconverter is disadvantageously increased by each of the components that are required to appropriately process the signals as well as the attendant bias circuitry. As such, a need also persists for downconverters to produce output signals in a more efficient manner. Additionally, it would be desirable for downconverters to be designed in such a simpler manner that required less space and could therefore be more compactly packaged.
An oscillator and upconverters and downconverters including the oscillator are provided, wherein the oscillator includes a transistor comprised of a semiconductor material having a wide bandgap of at least 2.0 eV for producing radio frequency (RF) output signals. Since the transistor is comprised of a semiconductor material having a wide bandgap, the resulting RF output signals can have much greater power than the output signals provided by conventional oscillators that include transistors formed of traditional semiconductor materials with relatively small bandgaps. The higher power is due to the larger voltage and current operation of wide bandgap transistors. The larger voltage operation is due to the higher breakdown field of wide band gap materials (approximately 2xc3x97106 V/cm) compared with that of relatively small band gaps (approximately 5xc3x97105 V/com). Thus, the oscillator as well as the upconverters and downconverters including the oscillator need not include the plurality of drivers, amplifiers or the like, nor any of the associated bias circuitry otherwise required by conventional upconverters and downconverters. The oscillator and the upconverters and downconverters including the oscillator can therefore produce output signals in a much more efficient manner than conventional upconverters and downconverters. In addition, upconverters and downconverters including the oscillator of the present invention generally have fewer components than traditional upconverters and downconverters and correspondingly require less space and can be packaged more compactly than the traditional upconverters and downconverters.
As mentioned above, an oscillator of the present invention includes a transistor comprised of a semiconductor material having a wide bandgap for producing an RF output signal. Preferably, the transistor is comprised of a semiconductor material, such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), silicon carbide (SiC) or boron nitride (BN), that has a bandgap of at least 2.0 electron volts (eV). Additionally, the transistor is preferably either a field effect transistor, a heterojunction bipolar transistor or a high electron mobility transistor.
In addition to the wide bandgap transistor, the oscillator includes a bias supply for providing a supply voltage and a supply current to the transistor. Additionally, the oscillator includes a tank circuit in electrical communication with the transistor for causing the transistor to produce an RF output signal once the transistor is provided with the supply voltage and the supply current. In this regard, the transistor includes first, second and third terminals and the tank circuit includes first and second reactances electrically connected to the first and second terminals of the transistor, respectively. As such, the tank circuit causes the transistor to be unstable, thereby producing an RF output signal. The tank circuit can also include a varactor electrically connected to a respective reactance. A control input is advantageously provided between the varactor and the respective reactance in order to tune the RF output signal.
According to one aspect of the present invention, a saturated upconverter is provided that includes an oscillator including a transistor formed of a wide bandgap semiconductor material that operates in saturation to produce an RF output signal in response to a supply voltage and a supply current. The saturated upconverter of this aspect of the present invention can also include an antenna in electrical communication with the oscillator for transmitting the RF output signal. As a result of the relatively high power of the RF output signal provider by the oscillator of the present invention, the antenna can transmit the RF output signal without requiring any additional amplification stages between the oscillator and the antenna. In order to provide isolation, the saturated upconverter can also include a switching element, such as a solid state power amplifier, disposed between the oscillator and the antenna.
According to another aspect of the present invention, a linear upconverter is provided that includes an oscillator including a transistor formed of a wide bandgap semiconductor material operating in a linear mode for producing an RF signal in response to a supply voltage and a supply current. The linear upconverter also includes a first mixer for mixing a signal having an intermediate frequency with a signal derived from the RF signal to produce a first signal. The linear upconverter also includes a second mixer for mixing the first signal produced by the first mixer with another signal derived from the RF signal to produce an output signal. As a result of the relatively high power of the RF signal provided by the oscillator of the present invention, the first and second mixers of the linear upconverter of this aspect of the present invention can receive the respective signals derived from the RF signal without requiring any additional amplification of the RF signal between the oscillator and the first and second mixers.
The linear upconverter of this aspect of the present invention can include other components, if so desired. In this regard, the linear upconverter can include a frequency alteration element disposed between the oscillator and a respective mixer for altering the frequency of the RF signal that is produced by the oscillator and provided to the respective mixer. The linear upconverter can also include an antenna for transmitting the output signal produced by the second mixer. Further, the linear upconverter can include a switching element disposed between the second mixer and the antenna to provide isolation. The linear upconverter can additionally include a first filter disposed between the first and second mixers and a second filter disposed between the second mixer and the antenna, both of which remove the signals derived from the RF signal.
According to another aspect of the present invention, a downconverter is provided that includes an oscillator having a transistor formed of a semiconductor material having a wide bandgap for producing RF signals in response to a supply voltage and a supply current. The downconverter also includes an antenna for receiving signals and a mixer for mixing the signals received by the antenna with signals derived from the RF signals. According to the present invention, however, the mixer receives the signals derived from the RF signals without requiring any additional amplification of the RF signals between the oscillator and the mixer. The downconverter of the present invention can also include one or more amplifiers between the antenna and the mixer to amplify the signals received by the antenna, if desired.
Therefore, upconverters, both saturated and linear, and downconverters are provided that can produce RF output signals of a desired frequency and power level in a more efficient manner. In this regard, the upconverters include an oscillator that has a transistor formed of a wide bandgap semiconductor material in order to produce an RF signal having a relatively high power. The upconverters therefore need not amplify the RF signals provided by the oscillator, thereby permitting many, if not all, of the drivers and amplifiers required by conventional upconverters to be eliminated. Thus, the upconverters of the present invention need not separately bias a plurality of drivers and amplifiers and can accordingly produce an RF output signal in a more efficient manner. Moreover, the upconverters of the present invention generally include fewer components and require less bias circuitry than conventional upconverters, thereby reducing the space requirements of the upconverters and permitting the upconverters to be packaged in a more compact manner.